1. Field of the Invention
The present invention relates to a microporous polyethylene film with good strength and stability at high temperature while having good permeability, more particularly to a microporous polyethylene film for a lithium secondary battery separator with high capacity and output.
2. Background of the Related Art
Microporous polyethylene films have been widely used for battery separators, separation filters and membranes for microfiltration, due to their chemical stability and excellent physical properties. In particular, separators for secondary battery require the highest grade quality along with the requirement for battery safety. Recently, with the improvement of capacity and output of secondary batteries, requirement of thermal stability of separators is becoming more rigorous. Especially, poor thermal stability of a separator for a lithium secondary battery may result in damage or deformation of the separator due to overheating of the battery, which may lead to short circuit of electrodes and fire.
Thermal stability of a battery is largely dependent on the strength of separator at high temperature.
Good separator strength at high temperature is needed to prevent damage of the separator at high temperature caused by the dendrites formed on the electrode during charge and discharge of a battery, and thereby to prevent short circuit between electrodes. Short circuit between electrodes results in overheating of the battery and, in severe cases, may lead to firing or explosion.
In addition to the thermal stability, long-term stability is also required for a battery. It is impossible to avoid heating during using battery. As a result, the separator experiences thermal deformation over a long period of time and loses its initial characteristics. In particular, permeability of the separator decreases due to thermal shrinkage and the migration of low molecular weight polyethylene included in the separator toward the surface at high temperature. As a result, the lifetime and capacity of the battery become poor. Therefore, thermal stability of a separator is very important in maintaining battery performance.
In order to improve thermal stability of a separator, U.S. Pat. No. 6,949,315 discloses a method of compounding ultrahigh molecular weight polyethylene with 5-15 wt % inorganic material, e.g., titanium oxide. However, this method is associated with the following problems. The use of the ultrahigh molecular weight polyethylene results in increased extrusion load, reduced extrusion compoundability, and decreased productivity due to insufficient stretching. Further, the addition of an inorganic material may lead to insufficient compounding and uneven quality and generating pinhole occurring therefrom. Besides, insufficient compatibility at the interface of the inorganic material and the polymer resin leads to poor film properties.
U.S. Pat. No. 5,641,565 discloses a method of improving thermal stability of a separator by using a highly heat-resistant resin. However, this technique requires the use of a ultrahigh molecular weight polymer having a molecular weight more than 1,000,000 in order to prevent deterioration of physical properties caused by the use of polyethylene with polypropylene and the addition of an inorganic material. Further, the process for removing the inorganic material by extracting makes the entire process complicated.
Japanese Patent Publication No. 1999-322989 aims at improving thermal stability of a separator by reducing shrinkage in a transverse direction. In this patent, stretching is carried out in the machine direction only or total stretch ratio is decreased to reduce thermal shrinkage in the transverse direction. As a result, low stretching ratio makes the improvement of physical properties poor the. The products described in the examples have a very low puncture strength of about 0.06-0.11 N/μm at room temperature. Although properties at high temperature are not presented, improvement of thermal stability at high temperature may be poor considering that puncture strength tends to decrease as temperature increases.
Although Japanese Patent Publication No. 2003-119306 discloses a separator having a shrinkage ratio less than 1%, separator strength was not measured at all and, in particular, strength at high temperature is not considered at all. Therefore, maximizing battery safety at high temperature may be difficult to be expected.
Further, all of the above-mentioned techniques, besides their weaknesses described above, do not consider the change of permeability after shrinking at high temperature at all. Therefore, it will be difficult to attain superior long-term lifetime and capacity of a battery.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.